I was looking into abbrev.el to figure out how to code up a change I
wanted to make (having undo undo the abbrev expansion without undoing
the self-insert-command that triggered the expansion), when I realized
to my surprise that abbrev is actually baked into the Emacs C core. 
This defies common sense: why can't abbrev.el be a standalone module
that uses post-self-insert-hook or post-command-hook?  I realize we're
not supposed to use hooks to communicate between components that are
part of GNU Emacs, but the alternative in this case seems unnecessarily
invasive.

(By the way: the comment in front of self-insert-command is no longer
valid; ISTR that dubious optimization was removed a while ago.)

=== modified file 'src/cmds.c'
--- src/cmds.c    2011-01-26 08:36:39 +0000
+++ src/cmds.c    2011-02-27 03:02:04 +0000
@@ -263,8 +263,6 @@
 
 static int nonundocount;
 
-/* Note that there's code in command_loop_1 which typically avoids
-   calling this.  */
 DEFUN ("self-insert-command", Fself_insert_command,
Sself_insert_command, 1, 1, "p",
        doc: /* Insert the character you type.
 Whichever character you type to run this command is inserted.